6 Answers
6

Famous question. Rabbi Yosef Karo in his commentary Bait Yosef (which would serve as a first draft of sorts of the Shulchan Aruch) writes not to eat fish with milk (Yoreh De'ah 87:3). His Ashkenazic counterpart Rabbi Moshe Isserles famously commented, "rabbi Karo mixed his milk with meat!" (נראה שנתערב לרב בית יוסף בשר בחלב; Darkhei Moshe, ibid), as the Talmudic quote regards fish with MEAT as being a health risk.

The fish-and-milk business was omitted from Rabbi Karo's later work, the Shulchan Aruch. So it's a question of how seriously we take the original comment in the Bait Yosef.

Non-Hassidic Ashkenazim simply follow Rabbi Isserles, that the whole statement was a mistake, and therefore there's no problem.

Among Sephardic Jews (who follow Rabbi Karo), and Hassidic Jews (who occasionally follow Rabbi Karo's stricter points), it varies from community to community. The mainstream Israeli "Eida Charedis" hechsher, which includes many Hassidic groups, will allow fish cooked in butter, but the Belz Hassidic hechsher won't. The Bukharian Sephardic practice may not be the Iraqi one, so you'd have to consult with your local hacham. Some rabbis also draw other differentiations, such as it's okay if they weren't cooked together (so yes bagel and lox, no tuna-noodle-casserole).

The Lubavitcher Rebbe writes in Reshimos: "The prohibition of eating Milk and Fish [mentioned] in the Bais Yosef is considered to be a mis-write. Nonetheless, we are careful [not to], therefore we add something (butter). This is an instruction from the Tzemach Tzedek".
The editors of Shaarei Halacha Uminhag write that it seems that adding butter to milk makes the danger go away.

To continue Shalom's answer, I would like to bring down the opinion of the Taz that says the Bet Yosef made a misprint and meant to write fish with meat. So wrote the Hida. However, Maran HaRab Obadia Yosef in Yechawe Daat writes "Yesh Lehimana" meaning that is prohibited according to Sephardim. Rab Obadia Yosef brings a Kula from the Kaf HaHaim that butter with fish one can be lenient. For Ashkenazim it is Mutar.

to all of the above learned commentors; you are all correct but not entirely. Not all ashkenazim are lenient and not all sefardim are stringent.
yes, it is based on a beis yosef and whether or not he mixed up the words meat and milk, but their were even rishonim who were machmir, including rabbeinu bachaye!
some feel its an actual medical issue and not the same as tzaraas mentioned by mixing meat and fish.
Moroccans are known to be lenient entirely, other sefardim bdieved; the ben ish hai was extremely stringent.
and although most ashkenazim are lenient some are mkpid due to the levish, pri megadim and g.maharsha being makpid for it.

Welcome to the Mi Yodeya (if the welcome's a bit late), and thanks for your answers. Specific sources (chapter and verse, so to speak) would be great if possible.
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msh210♦Aug 2 '12 at 13:24

i think this above answer is based on rabbi spitz's halacha column. i can tell you from past experience, if the original sources aren't listed in the article, just email the good rabbi and he will send you them - he most definitely has them! - yspitz@ohr.edu
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cmbAug 2 '12 at 17:07

Hello sam, welcome to Judaism.SE and thank you for this answer! To improve your answer, please consider linking to and/or summarizing that tshuva here. Also, please consider registering your account, which will give you access to more of the site's features. I hope to see you around!
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HodofHodApr 18 '12 at 5:37